**Core Concept**
A macrocytic anemia is characterized by an elevated mean corpuscular volume (MCV), indicating that the red blood cells are larger than normal. This type of anemia is often associated with impaired DNA synthesis, leading to an inability to produce normal-sized red blood cells.

**Why the Correct Answer is Right**
The most common cause of macrocytic anemia is a deficiency in vitamin B12 or folate. Vitamin B12 plays a crucial role in the synthesis of DNA, and a deficiency can lead to an inability to produce normal-sized red blood cells. In this patient, the impaired DNA synthesis is likely due to a vitamin B12 deficiency, which can cause a macrocytic anemia. The lack of intrinsic factor, a protein necessary for vitamin B12 absorption, can lead to a deficiency if the patient has a gastric atrophy or has undergone gastric surgery.

**Why Each Wrong Option is Incorrect**
**Option A:** Thalassemia major is a genetic disorder that affects hemoglobin production, leading to a microcytic anemia, not a macrocytic anemia.
**Option B:** Sickle cell anemia is a genetic disorder that affects hemoglobin production, leading to a microcytic anemia with sickled red blood cells, not a macrocytic anemia.
**Option C:** Iron deficiency anemia is a common cause of microcytic anemia, characterized by a decrease in hemoglobin and red blood cell size, not a macrocytic anemia.

**Clinical Pearl / High-Yield Fact**
A macrocytic anemia is often associated with a high neutrophil count, which can be a useful clue in the diagnosis. Additionally, a macrocytic anemia can be a sign of underlying gastrointestinal disease, such as gastric atrophy or celiac disease.

**Correct Answer:** C. Vitamin B12 deficiency.